Exposure to certain man-made and natural environmental agents poses a significant threat to human health. One of the major risks of exposure to harmful environmental agents is the development of cancer. Although the process of carcinogenesis is complex, it is clear that mutagenic activation of proto-oncogenes or inactivation of tumor suppressor genes play an important role. The long range goal of this research is to understand the role that cell proliferation plays in the process of mutation fixation in the intact animal. It is clear that many environmental agents interact with DNA and cause DNA damage which can be repaired by cellular DNA repair processes. Unrepaired DNA damage remaining at the time of DNA replication can lead to mutation fixation. Therefore, stimulation of enhanced cell proliferation would be expected to lead to enhanced mutagenesis. We propose to examine the influence of cell proliferation on mutation fixation in vivo by using the Big Blue transgenic mouse developed by Stratagene. This mouse carries an integrated bacteriophage lambda-based vector with the bacterial lacI gene as mutagenesis target. These mice will be treated with a chemical carcinogen (benzo[a]pyrene on the skin or by IP injection) with and without further treatments (phorbol 12-tetradecanoate 13-acetate on the skin or partial hepatectomy) to induce cell proliferation, and the frequency of mutations induced in the lacI target gene will be determined. Cell proliferation will be monitored by histological examination of the excised tissues and DNA adducts will be measured by P-postlabeling. Similar studies will be performed in a transgenic mouse line currently under development by Stratagene in which the normal control of cell proliferation has been altered (i.e., by a loss of p53 function combined with the mutagenesis target). These studies are expected to have important implications with respect to our understanding of the process of carcinogenesis and the role that environmental agents play in this process, in particular with regard to the synergistic effects of agents in complex mixtures and in relations to carcinogen testing protocols in which cell toxicity may enhance the apparent carcinogenic potential of certain agents.